Nitrate esters are commonly used in energetic formulations, such as smokeless powders, nitrate ester plasticized polyether (NEPE) propellants, and double base propellants. Nitrate esters are chemically unstable, decomposing slowly in a homolytic reaction. Byproducts of the homolytic reaction can catalyze further decomposition of the nitrate ester and damage a binder system in the energetic formulation if the byproducts accumulate.
To prevent autocatalysis, stabilizers are conventionally added to nitrate ester compositions. Such stabilizers include resorcinol, 2-nitrodiphenyl amine (2-NDPA), and N-methyl-4-nitroaniline (MNA). The stabilizer may limit or prevent autocatalysis by scavenging the byproducts of the homolytic reaction of the nitrate esters, thus controlling the overall decomposition rate. Over time, the concentration of a stabilizer in a nitrate ester composition may decrease as the nitrate ester decomposes. The stabilizer is added during manufacture in a quantity sufficient to preserve the physical and chemical integrity of the composition for the expected lifetime (shelf life or storage life) of the composition. In some applications, propellants may have an expected lifetime of several decades. Because the presence of crystallized stabilizer can adversely affect physical properties of the propellant, the stabilizer concentration at the time of manufacture is sufficiently low that the stabilizer remains solvated in the binder. Furthermore, use of stabilizers may be limited to only the amounts necessary to control decomposition of the nitrate ester. For example, a propellant may include about 0.5% stabilizer by weight at the time of production. The stabilizer concentration may gradually decrease during the lifetime of the propellant at a rate dependent upon storage conditions (e.g., temperature (including temperature cycling), humidity, vibration, etc.).
Contact with incompatible materials may accelerate the decomposition rate of nitrate ester compositions. This accelerated decomposition may significantly reduce the useful lifetime of nitrate ester propellants. For example, foreign object debris (FOD) can include materials of unknown composition in NEPE propellant mixes and finished rocket motors including such mixes. Procedures may be implemented to reduce contamination with FOD, such as FOD zones (i.e., areas where objects such as tools, packaging material, and debris are carefully tracked), ingredient screening, etc., but FOD remains an inherent risk of manufacturing. One partial solution to problems with FOD in nitrate ester compositions is to reformulate the compositions to make them more stable when placed in contact with materials likely to be FOD. That is, improving the resistance of nitrate ester compositions to FOD will reduce the decomposition rates of such compositions. Compatibility screening with ingredients (i.e., the ingredients purposefully added to propellants and motors) may also improve the stability of nitrate ester compositions. Nitrate ester compositions with higher stabilities may have longer useful lifetimes when exposed to incompatible materials.
NEPE propellants are conventionally formed by preparing a homogeneous liquid premix of nitroglycerin, polyethylene glycol (PEG), and MNA. The premix may be prepared separately and stored, if so desired. Solids, such as HMX, aluminum, ammonium perchlorate (AP), etc., are mixed with the premix to form a heterogeneous suspension or slurry. Curatives, generally including an isocyanate and a catalyst, are added to the suspension, and the propellant is cast and cured. Once cured, the propellant may have oxidizer crystals and powdered fuel held together in a rigid matrix of cross-linked binder.